For a drug to achieve its therapeutic effect, it needs to reach the target site in the body and further maintain the required level of blood or plasma concentration. Due to the short half-life of the active ingredient, many drugs need to be administered multiple times a day to maintain the desirable blood or plasma concentration. Furthermore, even with multiple administrations of such a drug per day, the blood or plasma concentrations of the active ingredient may still vary with time, i.e., at certain time points between administrations there are higher concentrations of the active ingredient than at other times. Thus, at certain time points of a 24-hour period, a patient may receive therapeutically effective amounts of the active ingredient, while at other time points the concentration of the active ingredient in the blood may fall below therapeutic levels. Additional problems with such drugs include that multiple dosing a day often adversely affects patient compliance with the treatment. Therefore, it is desirable to have a drug dosage form wherein the active ingredient is delivered in such a controlled manner that a constant or substantially constant level of blood or plasma concentration of the active ingredient can be achieved by one or at most two dosing per day. Accordingly, the present invention provides a controlled-release composition as described below.